This invention relates to crystal-oscillator circuits, and more particularly to oscillator buffers using current-mirror circuits without feedback.
Electronic systems and devices often must rely on precise clocks to sequence through states, process data, and perform other tasks. Crystal oscillators are often used to generate the precise clocks needed by these systems.
FIG. 1 is a diagram of a prior-art crystal oscillator. Crystal 12 oscillates at a fundamental frequency when a gain stage provides gain to start the crystal oscillating and then to maintain the oscillation. Crystal 12 is coupled between nodes X1 and X2, and is usually connected to other components such as inverter 14 by pins on an integrated circuit (IC). Inverter 14 inverts node X1 and drives node X2, acting as the gain stage and providing a 180-degree feedback signal to node X2 in relation to node X1. Feedback resistor 10 acts as a DC bias that biases inverter 14 in its gain region.
Capacitors 16, 18 provide a load capacitance to ground for nodes X1, X2. The value of capacitors 16, 18 can alter the frequency of oscillation of crystal 12. Any given crystal has a manufacturer-specified load capacitance that causes the crystal to oscillate at exactly the specified frequency. Larger capacitive loads on nodes X1, X2 slow down the oscillation, while smaller capacitive loads on nodes X1, X2 accelerate the oscillation.
The gain of inverter 14 must be large enough to provide a negative resistance to start oscillation. The negative resistance of inverter 14 is cancelled by the positive resistance of crystal 12, since the negative resistance provided by the inverter is equal to the series resistance of crystal. However as the amplitude of oscillations increase, the gain of inverter 14 is reduced by non-linearities of transistors in inverter 14.
The non-linearity of transistors in inverter 14 can distort the output waveform. Power loss and electro-magnetic interference (EMI) can increase. Higher-amplitude signals applied to crystal 12 can reduce its long-term stability. The gain of inverter 14 can vary with power-supply voltage Vdd, causing the oscillation frequency to vary with the power supply, which is undesirable.